1. Field of the Invention
This invention relates to an ink composition for a printer, and more particularly to a non-aqueous ink composition containing therein non-volatile solvent as a main material.
2. Description of the Related Art
Ink jet recording has been rapidly put into wide use since it realizes high-speed printing with relatively less noise. In an ink jet printer, highly flowable liquid ink is injected onto a recording medium through a fine nozzle, and the ink jet printers are broadly divided into an on demand type and a continuous type.
Inks for ink jet printers are broadly divided into an aqueous type and a non-aqueous type. The aqueous type inks generally comprise water-soluble dye such as acid dye, direct dye and basic dye dissolved in glycol solvent and water, and there has been a problem that ink jet printings by an aqueous type ink are generally poor in resistance to water.
On the other hand, non-aqueous type inks are excellent in resistance to water. The non-aqueous type inks are broadly divided into solvent type inks and oil type inks. Especially, various kinds of oil type inks have been proposed since the oil type ink is advantageous as ink for high-speed ink jet printer in that it is less apt to cause clogging of the nozzle and requires less frequent cleaning during printing. See, for instance, Japanese Unexamined Patent Publication Nos. 11(1999)-279467, 2000-168227 and U.S. Laid-Open No. 20020112641 and U.S. Pat. Nos. 5,723,545, 5,763,532, 5,837,787 and 6,121,383.
However, the oil type ink gives rise to a problem that the solvent in the ink is non-volatile and remains on the printings for a long time, and the residual solvent is apt to cause strike through (the printed part can be seen through from the back side of the printings) and/or smear marks (the residual solvent component oozes out from the printed part).
The problem of strike through or smear marks may be avoided by changing the solvent in the ink to volatile solvent. However, this approach will give rise to another problem that clogging of the nozzle becomes more apt to occur since the viscosity of the ink increases due to the volatile solvent and/or the dispersant is separated out in the solvent remaining near the nozzle, whereby the pigment in the ink is agglomerated. That is, when the ink is free from non-volatile solvent which can cause strike through and/or smear marks, clogging of the nozzle becomes more apt to occur, whereas when the ink contains non-volatile solvent, strike through and/or smear marks becomes more apt to occur though the problem of clogging of the nozzle can be avoided. In order to overcome both the problem of strike through and/or smear marks and the problem of clogging of the nozzle, the ink must have conflicting nature.